A project of three research groups at nanoGUNE at Donostia in San Sebastian, Spain, reports an innovative method to focus infrared light with tapered transmission lines to nanometer-size dimensions.  The key in that announcement is the method of transporting infrared energy by wire.  If the physics expressed in the paper work out to larger applications a major shift in engineering heat or thermal energy has dawned.  The work has been published at Nature Photonics.

The experiments conducted at nanoGUNE show that infrared light can be transported and nanofocused with miniature transmission lines, consisting of two closely spaced metal nanowires. While lenses and mirrors manipulate visible light in its form of a free-space propagating wave, transmission lines guide the infrared light in the form of a tightly bound surface wave.

The nanoGUNE team adapted the concept of a classic transmission line to the infrared frequency range. Transmission lines are specialized cables such as the familiar ‘coax’ to carry for example TV and radio frequency signals. An old and simple form consists of two metal wires running closely in parallel, also called ladder line that you might recall as flat 300-Ohm antenna cable used to connect the rooftop antennae to a TV. 300-Ohm line was widely used from the early days of broadcast television until cable TV demanded higher capacity of frequencies.  Applied at MHz frequencies, where typical wavelengths are in the range of centimeters to several meters, flat 300-Ohm antenna cable is a prime example illustrating transport of energy in waveguides of strongly subwavelength-scale diameter.

Infrared Wire Topography and Image. Click Image for more info.

The researchers developed experiments demonstrating that infrared light can be transported in the same way, by scaling down the size of the transmission lines to below 1 micrometer. To that end, they fabricated two metal nanowires connected to an infrared antenna. The antenna captures infrared light and converts it into a propagating surface wave for traveling along the transmission line.

By gradually reducing the width of the transmission line to a tapering shape, the researchers demonstrate that the infrared surface wave is compressed to a tiny spot at the taper apex with a diameter of only 60 nm. This tiny spot is 150 times smaller than the free-space wavelength, emphasizing the extreme subwavelength-scale focus achieved in the experiments. The researchers applied their recently introduced near-field microscopy technique (Schnell et al., Nano Lett. 10 3524 (2010)) to map the different electrical field components of the infrared focus with nanoscale resolution.

Connecting a transmission line to the antenna, the infrared light captured by the nanoantenna can be transported over significant distances and nanofocused in a remote place.  While we’re not primarily interested in focusing the infrared energy here, the team’s target is nanofocusing of infrared light with transmission lines because it has important implications in spectroscopy and sensing applications. Rainer Hillenbrand, leader of the Nanooptics Group at the nanoscience institute nanoGUNE said, “This opens new pathways for the development of infrared nanocircuits.”  Martin Schnell who performed the experiments offers, “It is amazing that the classical radiofrequency concepts still work at infrared frequencies. That is 30 THz!”

But the door is opened now- the prospect of transmitting the infrared spectrum by wire is an intensely interesting prospect.  Any thermal process yielding less than 50% efficiency is spending more on waste than the energy used, an appalling situation.  This is a new research and development field.

The applications that jump to mind are the Stratosolar idea of solar harvesting in the stratosphere, geothermal where one could go to hot locations and simply send the heat up by wire and the waste heat recovery market.  The stunning advantage is the heat isn’t hot until you start the conversion process to most likely electricity for example.  One could also dump into a large number of storage solutions as well. You’re sure to think of more.

The engineering potential is stunning. While the folks at nanoGUNE are hot on a worthwhile goal, the discovery offers lots of other connections for innovative engineering and product and process development.  Hats off to the Spaniards on this one!


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